Incompletely closed Nectar-cavities. 1 09 



stigma.) An insect that wishes to suck the nectar at 

 the bottom of the tube must insert its proboscis by the 

 margin of the stigma, and to do this must push away 

 the folded corolla, by which the stigma is closely en- 

 circled ; a task which is quite beyond the power of 

 any but strong insects.^ 



We come now to the second group of formations 

 mentioned at the beginning of this section, that, namely, 

 in which the passage that leads to the nectar is not 

 completely closed, but merely constricted — ^the con- 

 striction being effected by curvatures, swellings, dila- 

 tations, and aggregations of various parts. 



There are two ways in which such constrictions 

 benefit flowers. Either they completely exclude all 

 such nectar-hunting animals as owing to their small 

 dimensions would otherwise get at the honey without 

 fecundating the stigma with pollen brought from other 

 flowers, in which case they prevent an unprofitable 



1 [There are, it may be noticed, some flowers -where the nectar is 

 perfectly accessible to insects, though they will in getting at it 

 touch neither stigma nor anthers. Such, e.g., is the case with Mwrc- 

 graavia nepenthoides as described by Belt (Nicaragua, p. 129). 

 There are even plants, as the palosandre (ii. p. 130), where the 

 nectar is accessible to very small insects, while larger ones are 

 shut out. But in these cases the insects, though they touch 

 neither anther nor stigma, indirectly conduce to allogamy. For 

 as they are attracted' by the nectar, so in their turn they attract 

 humming-birds, which feed on them ; and these birds act as the 

 immediate agents of cross-fertiUsation. The larger insects are 

 excluded from the palosandre, as the consumption by them of the 

 honey would be a waste, since it is only small insects that furnish 

 food to humming-birds. — Editor.] 



